The notion of stability is central to the definition of personality traits, which are generally thought of as enduring tendencies or habitual patterns of behavior, thoughts, and emotions (McCrae & Costa, 2003), but stability does not imply immutability. Under normal circumstances, adult traits are largely stable, as indicated by high correlation coefficients computed for a group assessed twice on the same trait. These coefficients represent the average stability for a sample, but individuals vary in terms of their intra-individual stability. Recently, a longitudinal intra-individual approach was used to examine stabilities across two successive intervals traced within individuals. Individual stability coefficients from eight different longitudinal samples were computed to evaluate intra-individual (i.e., within-person) change in stability over time. Test-retest periods ranged from five to nine years. For both trait and profile (ipsative) stability, results indicate that intra-individual stability increases up to age 30 and then plateaus. Neither demographic variables (sex, ethnicity, education, and secular trends), nor the standing on the five major dimensions of personality, were predictors of change in trait stability. Contrary to results from studies of adolescents, personality maturity was unrelated to personality stability in adulthood. These findings support the notion that personality stability plateaus early in adulthood. Despite the relative stability of individual differences, in recent years we have extended the studies of mean-level change in personality by examining longitudinal trajectories in more diverse samples and cross-sectional trends across cultures. We have also focused on community or epidemiological investigations to study the causes of the modest changes that occur in personality traits in adulthood. In addition to health (see below), we are interested in the reciprocal influences of personality and life circumstances, such as job characteristics and retirement transition. Furthermore, we aim to delineate the neural and cognitive correlates of personality dimensions across longitudinal assessments. In health-related research, we have examined the association between personality traits and self-reports of physical health, as well as several biomarkers of mental and physical health. Below are some examples of this research. We examined the influence of personality traits on mean levels and age trends in 4 single-item measures of self-rated health: general rating, comparison to age peers, comparison to past health, and expectations for future health. Community-dwelling participants (N = 1,683) completed 7,474 self-rated health assessments over a period of up to 19 years. In hierarchical linear modeling analyses, age-associated declines differed across the 4 health items. Across age groups, high Neuroticism and low Conscientiousness, low Extraversion, and low Openness were associated with worse health ratings, with notable differences across the 4 health items. Furthermore, high Neuroticism predicted steeper declines in health ratings involving temporal comparisons. These findings suggest that personality is related to self-evaluation concerns, and that these effects are increasingly deleterious. Brain derived neurotrophic factor (BDNF) is among the most promising biomarkers of mood disorders. BDNF regulates synaptic plasticity and neurogenesis, and BDNF plasma and serum levels have been associated with depression and other psychiatric and neurodegenerative disorders. In clinical samples, circulating levels of BDNF are reported to be lower in depressed patients compared to controls and are reported to increase significantly with antidepressant treatment. In large community-based samples (N = 2,099), we have examined whether BDNF concentration in plasma and serum are associated with the personality trait of Neuroticism, a major risk factor for depression. We have found that serum but not plasma BDNF is associated with measures of psychological distress (Neuroticism, depressive symptoms). Among the markers of physical health, we have examined the association between personality traits and obesity, dyslipedemia, metabolic syndrome, arterial thickening, walking speed, muscle strength and inflammation. Below are summaries from one study on obesity and one on muscle strength. Personality traits contribute to health outcomes, in part through their association with major controllable risk factors, such as obesity. Body weight, in turn, reflects our behaviors and lifestyle and contributes to the way we perceive ourselves and others. Based on data from a large (N = 1,988) longitudinal study that spanned more than 50 years, we examined how personality traits are associated with multiple measures of adiposity and with fluctuations in body mass index (BMI). Using 14,531 anthropometric assessments, we modeled the trajectory of BMI across adulthood and tested whether personality predicted its rate of change. Measured concurrently, participants higher on Neuroticism or Extraversion or lower on Conscientiousness had higher BMI; these associations replicated across body fat, waist, and hip circumference. The strongest association was found for the impulsivity facet: Participants who scored in the top 10% of impulsivity weighed, on average, 11Kg more than those in the bottom 10%. Longitudinally, high Neuroticism and low Conscientiousness, and the facets of these traits related to difficulty with impulse control, were associated with weight fluctuations, measured as the variability in weight over time. Finally, low Agreeableness and impulsivity-related traits predicted a greater increase in BMI across the adult life span. BMI was mostly unrelated to change in personality traits. Personality traits are defined by cognitive, emotional, and behavioral patterns that likely contribute to unhealthy weight and difficulties with weight management. Such associations may elucidate the role of personality traits in disease progression and may help to design more effective interventions. Personality traits have also been linked to physical activity, partly because Extraverted individuals prefer activity and stimulation. Whether personality is directly related to physical function such as muscle strength, and more specifically to the retention of muscle function in the aging process, is an open question. Using cross-sectional data from 1,220 older adults, we associated personality as measured by the NEO-PI-R with knee strength. Controlling for age, sex, and ethnic minority status, participants high (0.5 SD above mean) in Neuroticism had lower muscle strength. Low Extraversion was associated with muscle strength when lifestyle practices were added to the model. Additionally, participants with the combination of high Neuroticism and either low Extraversion or low Conscientiousness had lower muscle strength than with either of those traits alone. Facet analyses suggest an important role for the Neuroticism components of depression and hostility. Results demonstrate that personality factors have important relationships with the maintenance of physical function in old age.